A vehicle seat is provided with a seat belt device for restraining the vehicle occupant to prevent the vehicle occupant from being thrown forward in case of a vehicle crash. Since such a seat belt device is desired not to obstruct the normal movement of the occupant and to be tightened only when a deceleration in excess of a prescribed value has been applied to the vehicle, there have been proposed a number of seat belt tightening devices which detect an impact of a vehicle crash through a displacement of an elastically supported pendulum caused by an inertia force acting thereon, ignite a propellant cartridge, and wind the seat belt by making use of the energy thus produced, for instance in copending U.S Pat. application Ser. No. 07/132,257, filed Dec. 14, 1987, now U.S. Pat. No. 4,864,086. U.S. Pat. No. 4,418,503 discloses a device for reducing an impact upon a vehicle occupant by fitting an air bag which can rapidly expand upon occurrence of a vehicle crash, in a steering wheel.
It has been customary to use those devices making use of the movement of a weight by its inertia as a crash detecting device for activating such a vehicle occupant protection device (refer to U.S. Pat. No. 4,580,810). Such a device must be capable of being activated without fail when subjected to a deceleration in excess of a prescribed level, but must not be inadvertently activated erroneously. On the other hand, since such a crash detecting device is only activated under special circumstances and cannot be tested in any simple fashion, an extremely high operation reliability is required.